The invention relates to a method for the production of wear-resistant sides for a keystone ring for internal combustion engines, in which a rectangular ring is first formed, consisting of a steel strip having a rectangular cross-section as the base material, having an inner and an outer circumference surface as well as an upper and a lower side, which ring is subsequently nitrided on all sides by means of a nitriding process, and subsequently brought into its final shape and geometry by means of machining of the surfaces, whereby the base material is exposed on the sides by means of the machining.
Piston rings having a trapezoid cross-section—ring width at the outer circumference surface greater than at the inner circumference surface—are preferably used in diesel engines as a top ring, whereby the related piston ring groove is also shaped as a trapezoid. Diesel engines tend towards deposits of hard residues that contain carbon, in the first—uppermost—piston ring groove, which can lead to seizing of the piston rings and therefore to functional failures. In general, seizing is prevented by means of the trapezoid configuration of the piston rings and the piston ring groove, and therefore the sealing function is guaranteed. In the production of rectangular rings for pistons of internal combustion engines, it is known to apply a nitride layer onto the entire surface of the ring, by means of a gas nitriding process, in order to harden the surface and thereby achieve better wear resistance. Such piston rings are described in DE 35 06 746 C2, EP 0 605 223 A1, or in JP 05060241 A, whereby the wear resistance of the working surface of the piston rings, in particular, is further increased by means of applying additional coatings, as is known from the aforementioned references and from DE 102 07 148 A1.
It is also known, in the production of keystone rings or double keystone rings, which are formed from rectangular rings, to subject the entire surface of the rings to a nitriding process. For forming or profile-shaping such a piston ring into a keystone ring, it is necessary to grind the sides accordingly, whereby the nitride layer is removed from the sides again, in whole or in part, depending on the geometrical requirements. Final profile-shaping takes place by means of further machining steps of the ring surfaces, with subsequent application of a wear protection coating of the working surface, as well as final machining. This production sequence is appropriately optimized with regard to the ability of the rings to be stacked, in other words simultaneous multiple machining in a package.
In engine operation, however, the keystone rings produced in this manner and inserted into steel pistons show that the ring sides increasingly demonstrate friction wear damage (fretting), particularly due to the high ignition pressures of 200 bar to 220 bar, whereby additional coking can also be found. This damage increases with an extension of the useful lifetime of the piston rings, and finally leads to failure. It has furthermore been shown that this damage occurs as a function of location, i.e. at different intensity on the ring sides, along the circumference.